(1) Field of Invention
This invention relates to subsea risers used to transport well fluids from the seabed to a surface installation such as an FPSO vessel or a platform. The invention relates particularly to systems for restraining movement of such risers under the action of currents or excursion of an FPSO.
Hybrid riser systems have been known for many years. Such systems use riser pipes, possibly of lined and coated steel, that extend upwardly from the seabed to near the surface. Flexible jumper pipes extend from there to the surface to add compliancy that decouples the more rigid riser pipes from surface movement induced by waves and tides. The riser pipes experience less stress and fatigue as a result, especially at the vulnerable sag bend near their touchdown point on the seabed.
More specifically, a hybrid riser system comprises a subsea riser support extending from a seabed anchorage to an upper end held buoyantly in mid water, at a depth below the influence of likely wave action. A depth of 250 m is typical for this purpose but this may vary according to the sea conditions expected at a particular location.
The riser support may comprise a hybrid riser tower or ‘HRT’ pivotably attached to the anchorage and held in tension by buoyancy at its upper end, or a riser support buoy tethered to the anchorage under tension. A riser support buoy is sometimes referred to by the acronym ‘BSR’, derived from the Portuguese term ‘bóia de suporte de riser’. That acronym will be used to identify riser support buoys in the description that follows.
Riser pipes extend from the seabed to the upper end region of the riser support. In the case of an HRT, the riser pipes will typically extend along the HRT as an upright bundle of generally parallel pipes. In the case of a BSR, the riser pipes will typically hang freely from, and splay away from, the BSR as steel catenary risers or ‘SCRs’. SCRs are a non-limiting example: other types of pipe are possible for the riser pipes.
Jumper pipes hanging as catenaries extend from the upper end region of the riser support to an FPSO or other surface installation. The FPSO is moored at a location above the riser support and spaced or offset horizontally from the riser support.
When viewed from above so that the arcuate shape of the jumper pipes and the depth of the riser support beneath the surface is not apparent, there is a general flow direction extending from the upper end region of the riser support toward the FPSO. The flow direction will be used to explain the present invention in more detail and is illustrated in FIGS. 2a, 3a and 4a of the accompanying drawings.
In the case of a BSR, when similarly viewed from above, the SCRs extend from the BSR in a direction generally opposed to the flow direction; optionally, the SCRs also diverge from each other moving away from the BSR.
Umbilicals and other pipes generally follow the paths of the riser pipes and jumper pipes to carry power, control data and other fluids.
In deep water, a surface installation such as an FPSO will usually have spread moorings. Spread moorings typically comprise four sets of mooring lines (each set being of say four to six mooring lines) with the sets radiating with angular spacing from the FPSO to anchors such as suction piles or torpedo piles embedded in the seabed. Such moorings can maintain the FPSO on location for several years at a fixed orientation or heading without ‘weathervaning’ rotation about a vertical axis. This minimal yaw movement means that there is no need for a turret structure or for swivel connections for fluids, power and control data. The connections are therefore advantageously simplified. Also, flexible riser pipes and umbilicals may simply be connected amidships along sides of the FPSO, which maximises the space available for those connections.
In a spread-moored arrangement, a riser system is typically accommodated between neighbouring sets of mooring lines of the FPSO. Space may be limited such that in extreme weather conditions, there is a potential for interference between the mooring lines of the FPSO and the riser supports and/or the riser pipes.
The potential for interference is greater still where a plurality of riser supports are combined with a single surface installation such as an FPSO, as more space is required for plural riser supports. Also, arrangements having a plurality of riser supports introduce the further risk of interference between neighbouring riser supports or between the riser pipes carried by those neighbouring riser supports.
It is desirable to stabilise riser supports against excessive movement in extreme conditions. The buoyancy that creates tension in a riser support is a stabilising factor; so too is the horizontal component of the force applied to the riser support by the jumper pipes. Also, where SCRs or other riser pipes hang from a BSR, the SCRs apply to a lesser extent a force to the BSR whose horizontal component is opposed to the horizontal component of the force applied to the BSR by the jumper pipes. This, too, helps to stabilise a BSR. However it may be desirable to apply other stabilising restoring forces to a riser support.
(2) Description of Related Art
GB 2346188, U.S. Pat. No. 6,595,725 and US 2006/0056918 disclose riser arrangements in which a plurality of riser supports are shared by a single surface installation. GB 2346188 discloses a row of HRTs whereas U.S. Pat. No. 6,595,725 and US 2006/0056918 each disclose two BSRs. All of those documents propose additional means for stabilising the riser supports but they work in very different ways—none of which are helpful for the purposes of the present invention.
GB 2346188 discloses interconnecting tethers between the riser towers near their upper ends. This interconnection is intended to limit differential movement between the neighbouring riser towers but it also allows—and indeed encourages—the whole row of riser towers to move together. So, there is nothing to prevent the row of towers colliding with any adjacent spread moorings. Also, the interconnecting tethers in GB 2346188 hang as shallow catenaries and so have negative buoyancy, which means that the riser towers will be pulled together by the tension in the tethers due to their weight. In practice, this will cause the riser towers to lean toward each other, thus increasing the risk of collision between neighbouring towers in extreme conditions. This is a particular risk with the towers at the ends of the row.
U.S. Pat. No. 6,595,725 discloses two riser supports but they are not grouped together: instead, one riser support is disposed to each side of a production facility floating above. The jumper pipes and riser pipes apply opposed stabilising forces to each riser support in directions parallel to the flow direction. Additionally, guy lines extend to the seabed from each riser support to prevent lateral movement due to water current. There is no practical risk of collision between the riser supports and there is space to avoid collision between the riser supports and spread moorings of the production facility. However, the arrangement would not be suitable for accommodating a group of two or more aligned riser supports between neighbouring sets of mooring lines of a spread-moored FPSO.
US 2006/0056918 discloses a weighted line between two riser supports but the weighted line only applies restoring forces parallel to the flow direction. As in U.S. Pat. No. 6,595,725 above, the riser supports are not grouped to one side of a surface installation floating above: instead, one riser support is disposed to each side of the surface installation. Again, therefore, there is no risk of collision between the riser supports and there would be space to avoid collision between the riser supports and spread moorings, if used.
If the riser supports of U.S. Pat. No. 6,595,725 or US 2006/0056918 were grouped to one side of the surface installation (not that there is any motivation or suggestion in those documents to adapt those proposals in that way), there would be a risk of collision between the riser supports and between the riser supports and spread moorings, if used.
It is against this background that the present invention has been devised.